Titanium dioxide is a well-known pigment and white opacifying agent. For example, titanium dioxide is used as a pigment in connection with coating formulations (including paint and ink formulations), paper compositions, polymer compositions and other products. Titanium dioxide pigments can be manufactured by either the sulfate process or the chloride process. Regardless of the manufacturing process used, the pigments are typically produced in powder form.
In the chloride process for manufacturing titanium dioxide, a dry titanium dioxide ore (for example, rutile or high-grade ilmenite) is fed into a chlorinator together with a carbon source (for example, coke) and chlorine and reacted at a high temperature to produce titanium tetrachloride (TiCl4) in vapor form. The gaseous titanium tetrachloride is condensed into liquid form and then purified to remove impurities therefrom. The purified titanium tetrachloride is then vaporized and reacted with oxygen in the vapor phase at a high temperature to produce titanium dioxide particles and gaseous reaction products. In order to achieve the necessary high temperature in the oxidizer, the titanium tetrachloride vapor and oxygen gas stream are usually preheated before being introduced into the oxidizer. Following the oxidation step, the titanium dioxide and gaseous reaction products are cooled and the titanium dioxide particles are recovered.
The recovered titanium dioxide particles are usually further processed before being sold and transported for use as a pigment. For example, depending on the anticipated end use application, the finishing process typically entails coating the titanium dioxide particles with one or more metal oxides to enhance the light scattering efficiency and durability of the pigment and impart other desired properties and characteristics thereto.
A metal chloride such as aluminum chloride is typically added to the titanium tetrachloride vapor in the oxidation reactor to incorporate a metal oxide into the crystalline lattice structure of the titanium dioxide. The metal oxide promotes rutilization of the titanium dioxide. It also enhances the durability of the finished pigment.
Metal chloride for use in a titanium halide vapor phase oxidation process is generally obtained in one of two ways: 1) by purchasing it from a commercial vendor; or 2) by generating it in situ. Each way has its advantages and disadvantages.
For example, pre-existing (e.g., purchased) metal chloride does not require the expense of the equipment necessary for generating the metal chloride in situ. However, pre-existing (e.g., purchased) metal chloride must first be dissolved into the condensed liquid titanium halide. This typically requires the titanium halide to be heated. The dissolution process can lengthen the time of the overall production cycle. Also, a commercial grade, pre-existing metal chloride such as aluminum chloride can contain impurities that react with the titanium halide and produce problematic deposits on the walls of the titanium halide vaporizer. Further, the mixture of the metal chloride and the titanium halide can be corrosive, which typically requires the oxidation reactor and related equipment to be lined with costly corrosion-resistant material.
The aluminum chloride can be generated in situ by various methods. For example, the metal chloride (for example, aluminum chloride) can be generated in a fluid bed reactor. In such a reactor, for example, a mixed stream of chlorine gas and titanium halide vapor from the vaporizer or preheater (depending on the sequence in which such equipment is used) can be introduced into a bed of solid metal (for example, solid aluminum) pellets in the bottom of the reactor and caused to flow vertically in the form of bubbles and interstitial gas. The bubbles and interstitial gas contact the metal pellets which cause the chlorine and metal to react to form metal chloride vapor. Blow over solid metal (for example, solid aluminum) particles can exit the fluid bed reactor as blow over. A sand bed is also typically included in the reactor in order to scour the surface of the aluminum or other metal pellets to break an aluminum oxide or other metal oxide coating that is generally inherently on the surfaces of the pellets.
The size of a fluid bed metal chloride generator is generally determined by the size of the metal particles allowed to exit the reactor as blow over and the exothermicity of the reaction between the metal and chlorine. The desired blow over metal particle size dictates the space velocity and transport disengagement height of the fluid bed. Usually, the particle size is chosen to be relatively small due to concerns of erosion and corrosion of downstream equipment. For example, the reaction of aluminum and chlorine is so exothermic that at typical addition levels, virtually the entire titanium halide vapor stream is run through the fluid bed metal chloride generator to serve as a heat sink for the process to keep the reaction temperature under control as dictated by the melting point of aluminum. Due to the exothermic nature of the reaction, the overall rate of addition of the aluminum and chlorine to the reactor is also a factor in determining the size of the reactor needed.
Normally, in order to produce a sufficient amount of metal chloride to provide the desired concentration of metal oxide in the titanium dioxide pigment, the fluid bed reactor is fairly large. A decrease in the particle size of the metal generally means an increase in the size of the reactor required. For example, in many cases, a fluid bed metal chloride generator on a titanium dioxide production line will be 16 feet in height and over three feet in diameter. Due to the corrosive nature of a titanium halide and metal chloride mixture, the reactor must generally be made of exotic alloys and be refractory lined. As a result, large fluid bed reactors can be costly due to capital, operational and maintenance costs.